Conventional radio frequency preselection techniques, for frequencies in the 0.1 to 1 Ghz range, require the use of narrowband (1 to 10 MHz) mechanically tuned filters to provide filtering of received RF signals and to provide appropriately filtered oscillator injection signals. These filters are usually combined with an active or passive mixer and an optional amplifier to complete the radio receiver front end.
Various attempts have been made to automate the tuning operation of the receiver's front end. For example, one conventional approach to automatic fine tuning employs an RF varactor tuned circuit, a frequency converter or mixer, a local oscillator, an auxiliary varactor tuned circuit, a phase discriminator and an amplifier arranged to provide the tuning control function. Wnile this approach may provide a form of automatic fine tuning, its phase discrimination scheme is undesireably complex, prone to temperature-induced instabilities, and limited in the frequency range over which it can provide tuning.
With the advent of new semiconductor technology, it is now possible to construct a narrowband filter which may be voltage tuned over more than half an octave without any appreciable degradation to the filter's characteristic. The use of these voltage tunable filters as injection and RF preselection filters, combined with an improved tuning control system, can significantly enhance the performance of existing receiver front ends.